Pump Element of a Hydraulic Unit for a Vehicle Brake System

ABSTRACT

A pump element of a hydraulic unit for a vehicle brake system is configured to deliver brake fluid via a piston that is mounted so as to reciprocate in a cylinder. The piston has a first and a second active surface, of which the first active surface is configured to cause brake fluid to be delivered from the cylinder during forward movement of the piston, and the second active surface is configured to cause brake fluid to be delivered from the cylinder during backward movement of the piston.

THE PRIOR ART

The invention relates to a pump element of a hydraulic unit for a vehicle brake system for delivering brake fluid by means of a piston mounted so as to reciprocate in a cylinder.

Pump elements of the type relevant here are also referred to as vehicle brake system piston pumps and have a reciprocating piston which is generally driven by means of a cam. Such pump elements, as used, for example, in systems having an electronic stability program (ESP), inherently deliver a volume of fluid, in particular brake fluid, per time unit which varies greatly over time. This highly uneven progression of the delivery volume exists on the suction side, but especially on the pressure side of the pump element, where it leads in some cases to pressure pulsations. These pressure pulsations cause, firstly, troublesome noises, referred to as Noise Vibration and Harshness (NVH). Secondly, the pressure peaks associated with the pressure pulsations are the cause of damage to components.

DISCLOSURE OF THE INVENTION

According to the invention there is provided a pump element of a hydraulic unit for a vehicle brake system for delivering brake fluid by means of a piston mounted so as to reciprocate in a cylinder, wherein the piston has a first and a second active surface, of which the first active surface causes brake fluid to be discharged from the cylinder during the forward movement of the piston and the second active surface causes fluid to be discharged from the cylinder during the backward movement of the piston.

The provision according to the invention of the two active surfaces has the effect of delivering brake fluid from the cylinder independently of whether the piston is moved into or out of the cylinder. The brake fluid therefore flows from the cylinder during both piston movements, whereby, in comparison to known pump elements, a more uniform volume flow of fluid is produced.

Known pump elements, by contrast, yield a more uneven fluid volume curve, because fluid is only aspirated as the piston is moved back from, or out of, the cylinder. During this suction phase of the pump element, which corresponds to, in particular, an angle of rotation of the associated cam of 180°, no volume is discharged by the piston of the pump element. The fluid is discharged only by a following further half-revolution of the cam, which causes the piston to be moved in, or forwards, into the cylinder. In this discharge phase a volume flow of fluid follows a sinusoidal curve with good approximation. This yields a curve of the pressure-side volume flow Q over the angle of rotation phi as shown in FIG. 3, the angle of rotation π being plotted on the x-axis 70 and the volume flow per time unit Q on the y-axis 72. The curve shows that no fluid is discharged during the first half-revolution of the cam which is illustrated, which causes the piston to be moved out and fluid therefore to be aspirated, and that fluid is discharged during the second half-revolution of the cam, which causes the piston to be moved in. Accordingly, the volume flow varies very greatly per time unit over the whole rotational movement of the cam, and is therefore uneven and pulsating.

A measure for evaluating this temporally uneven, pulsating volume flow is the degree of nonuniformity δ=(Q_(max)−Q_(min))/Q_(mittel)) where Q_(max) represents the maximum volume flow per time unit, Q_(min) the minimum volume flow per time unit and Q_(mittel) the mean value of the volume flow over time. For a known piston pump with a single pump element, the delivery curve of which is represented in FIG. 3, the degree of nonuniformity δ=π. This interrelationship applies to idealised conditions, that is, without taking account of a compressibility of the fluid or a volumetric efficiency.

In order to reduce the degree of nonuniformity during aspiration on a pump suction side, so-called stepped piston pumps are used in known hydraulic units, for example in ESP systems. Furthermore, additional balance pistons are known; these, as separate components, are also driven by a cam, and are typically arranged offset by 180° to the direction of movement of the piston pump. The hydraulic degree of nonuniformity on the pump pressure side is considerably reduced by the use of these additional balance pistons. However, additional installation space and additional material are used.

By contrast, the configuration of the pump element according to the invention produces a more uniform volume flow largely without the use of additional installation space and material.

Especially preferably, there is provided a pump element in which the first and second active surfaces are of different sizes. With this configuration a different force is advantageously required for moving the first active surface than for moving the second active surface. The second active surface is preferably smaller than the first active surface so that less force is required to move the second active surface than the first active surface. Furthermore, the first active surface is advantageously coupled non-positively to a cam while the second active surface is coupled non-positively to a return spring of the piston. With this coupling the first active surface, during a half-revolution of the cam which causes the forward movement of the piston, on the one hand acts compressively on a given volume of fluid or fluid volume and, on the other, acts deformingly on the return spring of the piston. In this case the following fluid volume V₁ is compressed: V₁=A₁×h₁, where A₁ is the area of the first active surface and h₁ the length of the distance travelled by the first active surface. The restoring force produced by the deformation of the return spring of the piston now presses the second active surface against the direction of movement of the first active surface during the backward movement of the piston. The second active surface therefore compresses, for a comparable distance travelled, a smaller fluid volume V₂, where V₂=A₂×h₂, A₂ is the area of the second active surface and h₂ is the length of the distance travelled by the second active surface.

There is preferably provided a pump element in which the first active surface is arranged to act in a first cylinder chamber and the second active surface in a second cylinder chamber, there being provided a first line through which brake fluid can flow from the first cylinder chamber into the second cylinder chamber as the piston is moved forward. Pumping of the fluid, in particular brake fluid, from the first cylinder chamber to the second cylinder chamber is therefore made possible.

The second cylinder chamber is advantageously smaller than the first cylinder chamber. Furthermore, the size ratio of the first to the second cylinder chamber is preferably substantially exactly equal to the size ratio of the first to the second active surface. Especially preferably, this size ratio is 2:1.

With this configuration, as the piston is moved forward by the force of the cam the first active surface acts compressively on a fluid contained in the first cylinder chamber. The compressed fluid, driven by the expansive urge of the fluid itself and by the force of the cam, flows through the first line into the second cylinder chamber. The second cylinder chamber is smaller than, in particular half as large as, the first cylinder chamber, so that only a portion of the fluid flowing in from the first cylinder chamber can be received therein. The remainder of the fluid flowing out of the first cylinder chamber is discharged from the pump element. In this way pumping of the fluid, in particular brake fluid, from the first cylinder chamber into the second cylinder chamber with simultaneous build-up of pressure of the fluid in the second cylinder chamber is advantageously made possible.

Furthermore, a first non-return valve which prevents brake fluid from flowing back from the second cylinder chamber into the first cylinder chamber is preferably arranged in the first line. Thus, the brake fluid can advantageously flow in only one direction, so that a delivery or transfer from the first to the second cylinder chamber without return flow losses is ensured. This has the advantage that one inlet is sufficient to allow the brake fluid to flow into the pump element.

In the pump element according to the invention the first line preferably passes through the piston. Configured in this way, the pump element is of very compact overall size with short flow paths. The short flow paths reduce friction of the fluid against an internal wall of the first line, lowering a heating of the fluid and adjacent components through friction, and the associated loss of kinetic energy. A further advantage is that, as a result of the lower consumption of material, the pump element according to invention can be produced very cost-effectively, unlike a pump element in which the first line passes along the outside of the cylinder.

There is preferably provided a pump element in which only one suction line for supplying brake fluid to the piston is provided. With only one suction line a very compact construction of the pump element is advantageously attained, saving material and space. The suction line is preferably connected to the first cylinder chamber, so that, during the backward or outward movement of the piston from the cylinder, brake fluid flows through the suction line into the first cylinder chamber until the latter is filled.

Especially preferably, only one pressure line for the discharge of brake fluid by the piston is provided in the pump element according to the invention, which pressure line is connected to the second cylinder chamber. With only one pressure line, the use of material and installation space for the pump element is advantageously further reduced and a still more compact construction is made possible. Furthermore, with the pressure line there is provided an outlet from the pump element through which brake fluid is displaced from the second cylinder chamber into a hydraulic system in order to perform work.

According to the invention, the process of displacing brake fluid from the second cylinder chamber takes place during both the forward and the backward movements of the piston. During the backward movement, that is, as the piston is moved out of the cylinder, on the one hand brake fluid is sucked through the suction line into the first cylinder chamber until the latter is filled. On the other, the brake fluid contained in the second cylinder chamber is simultaneously forced out of the second cylinder chamber into the pressure line by means of the second active surface. This means that, in contrast to known pump elements, during the backward movement of the piston brake fluid is not only sucked into the cylinder but is also forced out of the cylinder.

During the following forward movement, that is, as the piston is moved into the cylinder, the brake fluid is forced through the first line out of the first cylinder chamber and into the second cylinder chamber until the latter is filled. According to the invention, however, the second cylinder chamber is smaller than the first cylinder chamber, so that the surplus fluid volume flows out through the pressure line. In particular, the second cylinder chamber is only half as large as the first cylinder chamber, so that, in particular, half the fluid volume flows out through the pressure line. As the piston movement is repeated, that is, during the following backward movement of the piston, the first cylinder chamber is again filled with brake fluid through the suction line. At the same time, during this backward movement, the brake fluid still present in the second cylinder chamber is forced by means of the second active surface through the pressure line as a result of the coupling of the piston to the second active surface in the second cylinder chamber. This fluid is, in particular, the second half of the fluid volume which was aspirated during the preceding backward movement and temporarily stored in the second cylinder chamber during the preceding forward movement which followed said backward movement.

According to the invention, therefore, there is provided a delivery, or a delivery volume, of brake fluid from the pump element which extends over both directions of movement of the piston. Especially preferably, a particularly uniform volume flow of fluid is achieved by means of the halving of the second active surface in comparison to the first active surface, since, during the backward movement and during the forward movement of the piston half the fluid volume, and therefore an equally large fluid volume in each case, is discharged from the pump element.

Furthermore, there is preferably provided an intake chamber into which brake fluid is drawn during the forward movement of the piston. In order to supply brake fluid for this purpose, a suction line preferably leads into the intake chamber. The intake chamber is advantageously smaller than the first cylinder chamber and, especially advantageously, is half as large as the first cylinder chamber. As the piston is moved forward, or moved in, brake fluid flows through the suction line into the intake chamber, so that a priming of brake fluid for the following backward or outward movement of the piston is effected.

In addition, a second line, through which brake fluid is transferred from the intake chamber to the first active surface during the backward movement of the piston, is provided. Brake fluid thereby flows from the intake chamber into the first cylinder chamber, which is preferably larger than, especially preferably twice as large as, the intake chamber. That is to say that additional brake fluid is required in order to fill the first cylinder chamber and to compensate the suction produced thereby. The fluid volume required for this purpose is delivered, as a result of the suction produced in the first cylinder chamber, through the suction line into the intake chamber and from there through the second line into the first cylinder chamber. This takes place while the backward movement of the piston is still continuing.

Especially preferably, the second line passes through the piston, saving space and material, and in particular a second non-return valve is arranged in the second line. The second non-return valve advantageously prevents brake fluid from flowing back from the first cylinder chamber into the intake chamber, especially during the forward movement of the piston. During the forward movement of the piston, with the non-return valve closed, suction is again produced in the intake chamber, again drawing fluid through the suction line into the intake chamber. This means that, through the configuration of the pump element with the intake chamber according to the invention, there is produced an intake of brake fluid, or a volume intake, which extends over both directions of movement of the piston. Especially preferably, by halving the volume of the intake chamber as compared to the volume of the first cylinder chamber, an especially uniform volume flow of fluid is achieved, since half of the fluid volume, and therefore an equally large fluid volume in each case, is drawn into the pump element during the backward movement and during the forward movement of the piston.

To sum up, there is provided a compact pump element which can be produced at low cost and which makes possible a uniform delivery volume and a uniform intake volume of brake fluid during both directions of movement of the piston. The hydraulic degree of nonuniformity δ is therefore substantially reduced both on a pressure side and on the suction side of the pump, in particular reduced by half, so that δ=π/2. Undesired noise, such as so-called Noise Vibration and Harshness (NVH), is thereby advantageously reduced. In addition, the service life of components is especially advantageously lengthened by the avoidance of extreme pressure peaks.

An exemplary embodiment of the solution according to the invention is explained in more detail below with reference to the appended schematic drawings, in which:

FIG. 1 shows a longitudinal section through a schematically represented exemplary embodiment of the pump element according to the invention,

FIG. 2 shows a curve of a pressure-side volume flow of the exemplary embodiment, and

FIG. 3 shows a curve of a pressure-side volume flow of a pump element according to the state of the art.

FIG. 1 shows a pump element 10 of a hydraulic unit (not illustrated further) of a vehicle brake system for delivering brake fluid. The pump element 10 includes a cylinder 12 having a first cylinder chamber 14, a second cylinder chamber 16 and an intake chamber 18.

The pump element 10 further includes a piston 20 mounted so as to reciprocate in the cylinder 12. In order to be displaced, the piston 20 is coupled at one of its end faces to a cam 22 and at its opposite end face to a return spring 24, in a pressure-force transmitting manner in each case. Furthermore, the piston 20 has a first active surface 26 in the first cylinder chamber 14 and a second active surface 28 in the second cylinder chamber 16.

The second cylinder chamber 16 is connected to the first cylinder chamber 14 via a first line 30 which in the present example passes through the piston 20 and includes an associated non-return valve 32. In addition, a pressure line 34 for the discharge of brake fluid from the cylinder 12 by means of the piston 20 is provided on the second cylinder chamber 16. In order to supply brake fluid to the cylinder 12, a suction line attached to the intake chamber 18 is provided. A third active surface 38 of the piston 12 acts in the intake chamber 18. The intake chamber 18 is connected to the first cylinder chamber 14 via a second line 40 which, in the present embodiment, passes through the piston 20 and includes an associated non-return valve 42.

Because both the first line 30 and the second line 40 pass through the piston 20, a very compact construction of the pump element 10 is achieved.

In the present embodiment, the piston 20 is formed by a piston rod or piston cylinder 44 which has on one of its end faces a disk 46 which, in particular, is formed integrally with the piston cylinder 44. The piston cylinder 44 has a smaller diameter than the disk 46. The disk 46 has in turn an end face which, as a circular area, forms the first active surface 26 of the piston 20. Located opposite the first active surface 26 on the piston cylinder 44 is a reverse side of the disk 46 which forms an annular area and represents the third active surface 38 of the piston 20. In addition, the piston 20 includes a disk-shaped ring 48 which surrounds the piston cylinder 44 in a fluid-tight manner preferably at approximately the center of its longitudinal extent and which especially preferably is formed integrally with the piston cylinder 44. This disk-shaped ring 48 has two opposite, flat and equally large annular surfaces. The annular area of the disk-shaped ring 48 located opposite the first active surface 26 on the piston 20 serves as the second active surface 28. The active surfaces 26, 28 and 38, and therefore also the disk 46 and the disk-shaped ring 48, have an equal diameter. Despite the equal diameter, the active surfaces 26, 28 and 38 do not have equal areas. The first active surface 26 is a circular surface and therefore has a larger area than the second active surface 28 or the third active surface 38, which in each case are configured as annular surfaces having the same external diameter. In the present case the second active surface 28 and the third active surface 38 are preferably of equal size and especially preferably are each half as large as the first active surface 26.

In the present example the cylinder 12 comprises a first cylinder section 50 of a rectilinear circular cylinder having an end face 52 and an opposite annular surface 54 with an inner ring 56. In addition, a second cylinder section 58 of a rectilinear circular cylinder is provided as a hollow cylinder which is delimited by two opposite annular surfaces 60 and 62 having respective inner rings 64 and 66. The inner rings 56, 64 and 66 have an equal diameter. A third cylinder section 68 is coupled with a positive fit at one of its ends to the inner ring 66 at right angles to the annular surface 62, and at its other end with a positive fit to the inner ring 56 at right angles to the annular surface 54. The third cylinder section 68 has the same diameter as the inner rings 56 and 66. Configured in this way, the third cylinder section 68 connects the first cylinder section 50 to the second cylinder section 58. The second cylinder section 58 and the first cylinder section 50 have an equal internal diameter which is substantially equal to the diameter of the disk 46 and of the disk-shaped ring 48. By contrast, the internal diameter of the third cylinder section 68 is smaller, and substantially equal to the diameter of the piston cylinder 44. Especially preferably, the cylinder sections 50, 58 and 68 are configured as one piece.

The piston 20 as a whole is mounted so as to reciprocate along its longitudinal axis in a fluid-tight manner in the cylinder 12. It is mounted in such a way that the section of the piston 20 having the disk 46 is located in the first cylinder section 50 and the section of the piston 20 having the disk-shaped ring 48 is located in the second cylinder section 58. A section of the piston cylinder 44 disposed between the disk 46 and the disk-shaped ring 48 is located in the third cylinder section 68. Connected to the disk-shaped ring 48 is a section of the piston cylinder 44 which passes through the inner ring 64 of the annular surface 60 and is coupled non-positively via its end to the cam 22.

In a first movement phase during the forward movement of the piston 20, the piston 20 is moved into the cylinder 12 by a half-revolution of the cam 22. As this happens suction is produced which draws brake fluid from a container (not shown in detail) through the suction line 36 into the intake chamber 18, until the intake chamber 18 is filled with brake fluid. During this time the non-return valve 42 in the second line 40 connecting the intake chamber 18 to the first cylinder chamber 14 is closed. Simultaneously during the forward movement of the piston 20 the return spring 24 mounted in the first cylinder chamber 14 is compressed and tensioned by means of the first active surface 26. The restoring force of the return spring 24 produced thereby presses the piston 20, in conjunction with a second half-revolution of the cam 22, partially out of the cylinder 12 once more. This outward or backward movement of the piston 20 forms a second movement phase in which brake fluid flows from the intake chamber 18 through the second line 40, with the non-return valve 42 open, into the first cylinder chamber 14. The first cylinder chamber 14 has a larger volume than the intake chamber 18, in the present embodiment especially preferably a volume twice as large as that of the intake chamber 18. As a result, the entire brake fluid aspirated in the first movement phase flows into the first cylinder chamber 14 during the second movement phase. As a result of the suction produced thereby the missing volume of brake fluid is drawn simultaneously through the suction line 36 and in turn into the intake chamber 18. In this way an intake of brake fluid takes place in both phases of movement, that is, during both the forward and the backward movement of the piston 20. These two movement phases are repeated. Especially preferably, in the present embodiment, equal volumes of brake fluid are aspirated in each of the individual movement phases, whereby a uniform volume intake is advantageously produced.

In the piston movement, the second movement phase which has been described is followed once more by a first movement phase; that is, the piston 20 is again moved into the cylinder 12 by another half-revolution of the cam 22. During this forward movement brake fluid is aspirated as described. In addition, the brake fluid contained in the first cylinder chamber 14 is compressed by means of the first active surface 26 and forced via the first line 30, with the non-return valve 32 open, into the second cylinder chamber 16. The second cylinder chamber 16 has a smaller volume than the first cylinder chamber 14, especially preferably a volume half as large. As a result, only a part, preferably half, of the brake fluid forced out of the first cylinder chamber 14 can be taken up by the second cylinder chamber 16. The excess brake fluid is forced through the pressure line 34 out of the pump element 10 and thereby discharged.

In the second movement phase, or the backward movement, which again follows, the piston 20 again moves out of the cylinder 12. During this backward movement brake fluid is aspirated as described. In addition, the second active surface 28, with the non-return valve 32 closed, forces the brake fluid present in the second cylinder chamber 16 through the pressure line 34 and out of the cylinder chamber 16. The volume of brake fluid thereby discharged is preferably the second half of the brake fluid volume displaced from the first cylinder chamber 14 in the forward movement.

Together with the aspiration described, the discharge of brake fluid also takes place during both movement phases, that is, during both the forward and the backward movement of the piston 20. This discharge accompanying both movement directions of the piston 20 advantageously effects a uniform volume delivery of brake fluid. Especially preferably, with the present embodiment equal volumes of brake fluid are discharged in each of the individual movement phases, producing an especially uniform volume delivery.

The especially uniform volume delivery of the pump element 10 according to the present invention is represented in the curve of FIG. 2. As a comparison, the curve of FIG. 3 shows a nonuniform volume delivery in known pump elements.

In the curves, the discharged volume of brake fluid per time unit, the so-called pressure-side volume flow 70, is plotted on the y-axis. The pressure-side volume flow is dependent on an angle of rotation 72 which describes the movement of the cam 22, and therefore the movement of the piston 20. This angle of rotation 72 is therefore plotted on the x-axis.

In known pump elements (FIG. 3) brake fluid is aspirated only as the piston is moved out of the cylinder. During this suction phase, or the first movement phase of the piston, which comprises an angle of rotation 72 of the associated cam of 180° or π, no volume of brake fluid is discharged by the piston. The pressure-side volume flow 70 therefore equals zero. Fluid is discharged only by a further half-revolution of the cam, comprising a further rotation angle 72 of 180° or π, which then follows and causes the piston to be moved into the cylinder. In this delivery phase, or second movement phase of the piston, the pressure-side volume flow 70 follows a sinusoidal curve with good approximation. The curve in FIG. 3 shows that the pressure-side volume flow 70 per time unit varies very greatly over the whole rotational movement of the cam, and therefore is nonuniform and pulsating.

A measure for evaluating this nonuniform, pulsating volume flow over time is the degree of nonuniformity δ=(Q_(max)−Q_(min))/Q_(mittel)) where Q_(max) represents the maximum volume flow per time unit 74, Q_(min) the minimum volume flow per time unit 76 and Q_(mittel) the mean value of the volume flow over time. For a known piston pump with a single pump element, the delivery curve of which is represented in FIG. 3, the degree of nonuniformity δ=π. This interrelationship applies to idealised conditions, that is, without taking account of a compressibility of the fluid or of a volumetric efficiency.

In contrast to the above, in the exemplary embodiment of the pump element 10 according to the invention which has been described the volume curve of of the pressure-side volume flow 70 represented in FIG. 2 is achieved. Brake fluid is discharged as the piston 20 is moved both out and in, and during each associated half-revolution of the cam through the rotation angle 72 of 180° or π. Especially preferably, in the present example equal volumes of brake fluid are discharged in both movement phases of the piston 20. As a result, the degree of nonuniformity δ in the present exemplary embodiment is π/2, and is halved in comparison to the known pump element represented in FIG. 3, idealised conditions being assumed.

Corresponding volume ratios apply to the intake of brake fluid. 

1. A pump element of a hydraulic unit for a vehicle brake system, configured to deliver brake fluid via a piston mounted so as to reciprocate in a cylinder, wherein the piston has a first and a second active surface, of which the first active surface is configured to cause brake fluid to be discharged from the cylinder during forward movement of the piston and the second active surface is configured to cause brake fluid to be discharged from the cylinder during backward movement of the piston.
 2. The pump element as claimed in claim 1, wherein the first and second active surfaces are configured to be of different sizes in relation to each other.
 3. The pump element as claimed in claim 1, wherein the first active surface is located so as to act in a first cylinder chamber and the second active surface is located so as to act in a second cylinder chamber; and the pump element includes a first line, through which brake fluid can flow from the first cylinder chamber into the second cylinder chamber during the forward movement of the piston.
 4. The pump element as claimed in claim 3, wherein a first non-return valve is positioned in the first line.
 5. The pump element as claimed in claim 3, wherein the first line passes through the piston.
 6. The pump element as claimed in claim 1, wherein the pump element includes only one suction line that is configured to feed brake fluid to the piston.
 7. The pump element as claimed in claim 3, wherein: the pump element includes only one pressure line that is configured to discharge brake fluid via the piston; and the one pressure line is connected to the second cylinder chamber.
 8. The pump element as claimed in claim 1, wherein the pump element includes an intake chamber into which brake fluid is drawn during the forward movement of the piston.
 9. The pump element as claimed in claim 8, wherein the pump element further includes a second line, through which brake fluid is transferred from the intake chamber to the first active surface during the backward movement of the piston.
 10. The pump element as claimed in claim 9, wherein the second line passes through the piston. 